Methods of preparing polyhemiaminals and polyhexahydrotriazines

ABSTRACT

Polyhexahydrotriazine (PHT) film layers are formed by a process comprising heating a first mixture comprising i) a solvent, ii) paraformaldehyde, and iii) a diamine monomer comprising two primary aromatic amine groups at a temperature of about 20° C. to less than 150° C. This heating step forms a stable polyhemiaminal (PHA) in solution, which can be cast on a surface of a substrate, thereby forming an initial film layer comprising the PHA. The initial film layer is heated at a temperature of 180° C. to about 280° C., thereby converting the PHA film layer to a PHT film layer. Young&#39;s moduli of about 8 GPA to about 14 GPA have been observed for the PHT film layers.

BACKGROUND

The present invention relates to methods of preparing polyhemiaminalspolyhexahydrotriazines, and more specifically to preparingpolyhemiaminals and polyhexahydrotriazines derived from aromaticdiamines, and films therefrom.

Commercially important nitrogen-containing polymers include polyamides(nylon), polyimides (Kaplon, UPILEX, VTEC), and polyamines. Betweenthese three classes of materials, nitrogen-rich polymers haveapplications in adhesives, semiconductors, automotive components,electronics, sporting goods, coatings, bottles, foams, yarns, plumbingparts, paints, and hospital equipment, to name a few. Though widelyused, nitrogen-containing polymers can be flexible, hygroscopicmaterials sensitive to acids, bases and oxidants, which prevents theiruse in other applications.

A need exists for chemically resistant nitrogen-containing polymers thathave high rigidity and high tensile strength.

SUMMARY

Accordingly, A polyhexahydrotriazine (PHT) is disclosed, comprising:

a plurality of trivalent hexahydrotriazine groups having the structure

and

a plurality of divalent bridging groups of formula (2):

wherein

L′ is a divalent linking group selected from the group consisting of*—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, and combinations thereof,wherein R′ comprises at least 1 carbon and R″ comprises at least onecarbon,

each starred bond of a given hexahydrotriazine group is covalentlylinked to a respective one of the divalent bridging groups, and

each starred bond of a given bridging group is linked to a respectiveone of the hexahydrotriazine groups.

Also disclosed is a method, comprising:

forming a reaction mixture comprising a i) solvent, ii)paraformaldehyde, and iii) a monomer comprising two primary aromaticamine groups; and

heating the reaction mixture at a temperature of 150° C. to about 280°C., thereby forming a polyhexahydrotriazine (PHT).

Another method is disclosed, comprising:

forming a first mixture comprising a i) solvent, ii) paraformaldehyde,and iii) a monomer comprising two primary aromatic amine groups;

heating the first mixture at a temperature of about 20° C. to about 120°C., thereby forming a second mixture comprising a polyhemiaminal (PHA);

casting the second mixture on a surface of a substrate, thereby formingan initial film layer disposed on the surface, the initial film layercomprising the PHA and the solvent; and

heating the initial film layer at a temperature of 150° C. to about 280°C., thereby forming a second film layer comprising apolyhexahydrotriazine (PHT).

Further disclosed is a film layer comprising the above-describedpolyhexahydrotriazine (PHT).

Also disclosed is a device comprising the above-described film layer.

Another device is disclosed, comprising:

a layer comprising a polyhemiaminal (PHA) disposed on a surface of asubstrate, the PHA comprising

a plurality of trivalent hemiaminal groups having the structure

and

a plurality of divalent bridging groups of formula (2):

wherein

L′ is a divalent linking group selected from the group consisting of*—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, and combinations thereof,wherein R′ comprises at least 1 carbon and R″ comprises at least onecarbon,

each starred bond of a given hemiaminal group is covalently linked to arespective one of the divalent bridging groups, and

each starred bond of a given bridging group is linked to a respectiveone of the hemiaminal groups.

The above-described and other features and advantages of the presentinvention will be appreciated and understood by those skilled in the artfrom the following detailed description, drawings, and appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a series of cross-sectional layer diagrams illustrating thepreparation of a polyhemiaminal (PHA) film.

FIG. 2 is a series of cross-sectional layer diagrams illustrating thepreparation of a polyhexahydrotriazine (PHT) film.

FIG. 3 is a ¹H NMR spectrum of N,N-dimethyl-p-phenylenediamine ind₆-DMSO.

FIG. 4 is a ¹H NMR spectrum of a hemiaminal formed by the reaction ofN,N-dimethyl-p-phenylenediamine with paraformaldehyde.

FIG. 5 is a ¹H NMR spectrum of crude4,4′,4″-(1,3,5-triazinane-1,3,5-triyl)tris(N,N-dimethylaniline) formedby the reaction of N,N-dimethyl-p-phenylenediamine with paraformaldehyde(Example 1).

FIG. 6 is a ¹H NMR spectrum of purified4,4′,4″-(1,3,5-triazinane-1,3,5-triyl)tris(N,N-dimethylaniline) formedin Example 1.

FIG. 7 is a solid state ¹³C NMR spectrum of the polyhemiaminal formed inExample 6.

FIG. 8 is a differential scanning calorimetry (DSC) scan for thepolyhemiaminal film of PHA-1 (Example 6) after curing at 50° C. Theglass transition temperature (Tg) is 125° C.

FIG. 9 is a DSC scan of the polyhemiaminal film PHA-1 (Example 6) curedat 50° C., then heated to 200° C. for 1 hour prior to the analysis. Theglass transition temperature (Tg) is 222.27° C.

FIG. 10 is a DSC scan of the polyhexahydrotriazine film PHT-1 (Example13), which was cured at 200° C. for 1 hour prior to the analysis. Theglass transition temperature (Tg) is about 192° C., close to the curingtemperature.

FIG. 11 is a graph showing DMA scans for storage modulus (bottom twocurves) and thermogravimetric analysis (TGA, top 2 curves) scans formass loss of films prepared with 4,4′-oxydianiline (ODA) under varyingfilm cure conditions.

FIG. 12 is an atomic force microscope (AFM) image of the polyhemiaminalfilm PHA-1 (Example 6) cured at 50° C.

FIG. 13 is an AFM image of the polyhexahydrotriazine film PHT-1 (Example13) was cured at 200° C.

FIG. 14 is a ¹H NMR spectrum of 4,4′-(9-fluorenylidene)dianiline (FDA)monomer in d₆-DMSO.

FIG. 15 is a ¹H NMR spectrum of the methylene regions of the reactionproduct of FDA with paraformaldehyde in NMP (¹H NMR aliquots in d₆-DMSOsolvent) at different temperatures: (A) at 50° C. after 2 hours,integration of a:b=1:6 (B) at 200° C. after 2 hours.

FIG. 16 is a GPC trace using THF as eluent of the polymerization productof FDA with 2.5 equiv paraformaldhyde for 2 hours at 50° C. in 0.33 MNMP. The polyhemiaminal was purified by precipitation in water.Mw=10,576, Mn=3,588, and PDI=2.95.

FIG. 17 is a GPC trace using THF as eluent of the polymerization productof FDA with 2.5 equiv paraformaldhyde for 2 hours at 200° C. in 0.33 MNMP. Mw=48,910; Mn=8,174; PDI=5.9.

FIG. 18 is an XRD pattern obtained for crystalline 4,4′-oxydianiline(ODA) monomer.

FIG. 19 is an XRD pattern showing diffuse scattering observed for thepolyhexahydrotriazine (PHT) film formed with ODA.

FIG. 20 is a drawing of the cube-corner geometry of the nanoindenterprobe. The probe has a pyramid angle of 90° and an end radius of about50 nm. Angle a=90°.

FIG. 21 is a graph showing the reduced modulus and hardness data forExample 6.

FIG. 22 is a graph showing the reduced modulus and hardness data forExample 13.

FIG. 23 is a graph showing the reduced modulus and hardness data forExample 14.

FIG. 24 is a graph showing the reduced modulus and hardness data forExample 15.

FIG. 25 is a reaction diagram showing the formation of ahexahydrotriazine from formaldehyde and methylamine based on densityfunctional theory (DFT) calculations.

FIG. 26 is a free energy diagram for the formation of hexahydrotriazinefrom formaldehyde and methylamine based on density functional theory(DFT) calculations.

DETAILED DESCRIPTION

Methods are disclosed for preparing polyhemiaminals (PHAs) andpolyhexahydrotriazines (PHTs) by the reaction of aromatic diamines andparaformaldehyde. The PHAs and PHA films are stable intermediates in thepreparation of the PHTs and PHT films, respectively. The PHAs aregenerally prepared at a temperature of about 20° C. to about 120° C.,more, preferably at about 20° C. to about 100° C., and most preferablyat about 40° C. to about 60° C. The PHAs form films when cast from apolar aprotic solvents (e.g., NMP), and the PHA films are stable at atemperature of about 20° C. to less than 150° C. The PHA films can havea Young's modulus of about 6 GPa, which is exceptionally high for anorganic film.

The PHT films are formed by thermally treating a PHA film at atemperature of at least 150° C., preferably about 165° C. to about 280°C., more preferably about 180° C. to about 210° C., and most preferablyabout 190° C. to about 210° C. for a period of time of about 1 minute toabout 24 hours, and more preferably about 1 hour. The PHT films can havehigh heat resistance as measured by dynamic mechanical analysis (DMA).The PHT films can also have a high Young's modulus as measured bynanoindentation methods. In some instances, the Young's modulus of a PHTfilm can have a value in a range of about 8 GPa to about 14 GPa,exceeding that of bone (9 GPA).

Herein, a polyhemiaminal (PHA) is a crosslinked polymer comprising i) aplurality of trivalent hemiaminal groups of formula (1):

covalently linked to ii) a plurality of bridging groups of formula (2):

wherein y′ is 2 or 3, and K′ is a divalent or trivalent radicalcomprising at least one 6-carbon aromatic ring. Herein, starred bondsrepresent attachment points to other portions of the chemical structure.Each starred bond of a given hemiaminal group is covalently linked to arespective one of the bridging groups. Additionally, each starred bondof a given bridging group is covalently linked to a respective one ofthe hemiaminal groups.

As an example, a polyhemiaminal can be represented herein by formula(3):

In this instance, each K′ is a trivalent radical (y′=3) comprising atleast one 6-carbon aromatic ring. It should be understood that eachnitrogen having two starred wavy bonds in formula (3) is a portion of adifferent hemiaminal group.

The structure of formula (3) can also be represented using the notationof formula (4):

wherein x′ is moles and each bridging group K′ is a trivalent radical(y′=3 in formula (2)) comprising at least one 6-carbon aromatic ring. Itshould be understood that each starred nitrogen bond of a givenhemiaminal group of formula (4) is covalently linked to a respective oneof the bridging groups K′. Additionally, each starred bond of a givenbridging group K′ of formula (4) is covalently linked to a respectiveone of the hemiaminal groups.

Non-limiting exemplary trivalent bridging groups include:

The bridging groups can be used singularly or in combination.

The remainder of the description discusses divalent bridging groups K′.It should be understood that the methods and principles below also applyto trivalent linking groups.

Polyhemiaminals composed of divalent bridging groups K′ can berepresented herein by formula (5):

wherein K′ is a divalent radical (y′=2 in formula (2)) comprising atleast one 6-carbon aromatic ring. Each nitrogen having two starred wavybonds in formula (5) is a portion of a different hemiaminal group.

More specific divalent bridging groups have the formula (6):

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ and R″ independently comprise at least1 carbon. In an embodiment, R′ and R″ are independently selected fromthe group consisting of methyl, ethyl, propyl, isopropyl, phenyl, andcombinations thereof. Other L′ groups include methylene (*—CH₂—*),isopropylidenyl (*—C(Me)₂-*), and fluorenylidenyl:

Polyhemiaminals composed of divalent bridging groups of formula (6) canbe represented herein by formula (7):

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ and R″ independently comprise at least1 carbon. Each nitrogen having two starred wavy bonds in formula (7) isa portion of a different hemiaminal group.

The polyhemiaminal of formula (7) can also be represented by thenotation of formula (8):

wherein x′ is moles, and L′ is a divalent linking group selected fromthe group consisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ and R″ independently comprise at least1 carbon. Each starred nitrogen bond of a given hemiaminal group offormula (8) is covalently linked to a respective one of the bridginggroups. Additionally, each starred bond of a given bridging group offormula (8) is covalently linked to a respective one of the hemiaminalgroups.

The hemiaminal groups can be bound non-covalently to water and/or asolvent. A non-limiting example is a hemiaminal group that is hydrogenbonded to two water molecules as shown in formula (9):

In an embodiment, a polyhexahydrotriazine (PHT) is a crosslinked polymercomprising i) a plurality of trivalent hexahydrotriazine groups offormula (10):

covalently linked to ii) a plurality of divalent bridging groups K′(y′=2) of formula (2). Each starred bond of a given hexahydrotriazinegroup of formula (10) is covalently linked to a respective one of thebridging groups K′. Additionally, each starred bond of a given bridginggroup is covalently linked to a respective one of the hexahydrotriazinegroups.

For PHTs comprising bridging groups of formula (6), thepolyhexahydrotriazine is represented herein by formula (11):

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ and R″ independently comprise at least1 carbon. Each nitrogen having two starred wavy bonds in formula (11) isa portion of a different hexahydrotriazine group.

The polyhexahydrotriazine is also represented herein by the notation offormula (12):

wherein x′ is moles, L′ is a divalent linking group selected from thegroup consisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ comprises at least 1 carbon and R″comprises at least one carbon. Each starred bond of a givenhexahydrotriazine group of formula (12) is covalently linked to arespective one of the bridging groups. Additionally, each starred bondof a given bridging group of formula (12) is covalently linked to arespective one of the hexahydrotriazine groups.

The polyhexahydrotriazine can be bound non-covalently to water and/or asolvent (e.g., by hydrogen bonds).

Exemplary non-limiting divalent bridging groups include:

and combinations thereof.

The PHA and PHT can further comprise monovalent aromatic groups(referred to herein as diluent groups), which do not participate inchemical crosslinking and therefore can serve to control the crosslinkdensity as well as the physical and mechanical properties of the PHA andPHT polymers. Monovalent diluent groups have a structure according toformula (8), formula (9), formula (10), and/or formula (11):

wherein W′ is a monovalent radical selected from the group consisting of*—N(R¹)(R²), *—OR³, —SR⁴, wherein R¹, R², R³, and R⁴ are independentmonovalent radicals comprising at least 1 carbon. The starred bond islinked to a nitrogen of a hemiaminal group or a hexahydrotriazine group.

Non-limiting exemplary diluent groups include:

wherein the starred bond is linked to a nitrogen of a hemiaminal groupor a hexahydrotriazine group. Diluent groups can be used singularly orin combination.

A method of preparing a polyhemiaminal (PHA) comprising divalentbridging groups comprises forming a first mixture comprising i) amonomer comprising two or more primary aromatic amine groups, ii) anoptional diluent monomer comprising one aromatic primary amine group,iii) paraformaldehyde, and iv) a solvent. The first mixture is thenpreferably heated at a temperature of about 20° C. to about 120° C. forabout 1 minute to about 24 hours, thereby forming a second mixturecomprising the PHA. In an embodiment, the monomer comprises two primaryaromatic amine groups.

The mole ratio of paraformaldehyde: total moles of primary aromaticamine groups (e.g., diamine monomer plus optional monoamine monomer) ispreferably about 1:1 to about 1.25:1, based on one mole ofparaformaldehyde equal to 30 grams.

Non-limiting exemplary monomers comprising two primary aromatic aminegroups include 4,4′-oxydianiline (ODA), 4,4′-methylenedianiline (MDA),4,4′-(9-fluorenylidene)dianiline (FDA), p-phenylenediamine (PD),1,5-diaminonaphthalene (15DAN), 1,4-diaminonaphthalene (14DAN), andbenzidene, which have the following structures:

Non-limiting exemplary diluent monomers includeN,N-dimethyl-p-phenylenediamine (DPD), p-methoxyaniline (MOA),p-(methylthio)aniline (MTA), N,N-dimethyl-1,5-diaminonaphthalene(15DMN), N,N-dimethyl-1,4-diaminonaphthalene (14DMN), andN,N-dimethylbenzidene (DMB), which have the following structures:

The diluent monomer can be used in an amount of 0 mole % to about 75mole % based on total moles of monomer and diluent monomer.

The solvent can be any suitable solvent. Preferred solvents includedipolar aprotic solvents such as, for example, N-methyl-2-pyrrolidone(NMP), dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF),N,N-dimethylacetamide (DMA), Propylene carbonate (PC), and propyleneglycol methyl ether acetate (PGMEA). Most preferably, the solvent isNMP.

A method of preparing a polyhexahydrotriazine (PHT) having divalentbridging groups comprises forming a first mixture comprising i) amonomer comprising two aromatic primary amine groups, ii) an optionaldiluent monomer comprising one aromatic primary amine group, iii)paraformaldehyde, and iv) a solvent, and heating the first mixture at atemperature of at least 150° C., preferably about 165° C. to about 280°C., thereby forming a second mixture comprising a polyhexahydrotriazine.The heating time at any of the above temperatures can be for about 1minute to about 24 hours.

Alternatively, the PHT can be prepared by heating the solutioncomprising the PHA at a temperature of at least 150° C., preferablyabout 165° C. to about 280° C. even more preferably at about 180° C. toabout 220° C., and most preferably at about 200° C. for about 1 minuteto about 24 hours.

Also disclosed is a method of preparing a polyhemiaminal film,illustrated in the cross-sectional layer diagrams of FIG. 1. A mixturecomprising a polyhemiaminal and a solvent prepared as described above isdisposed on a surface 12 of a substrate 10, thereby forming structure 20comprising an initial film layer 22 comprising the polyhemiaminal,solvent and/or water disposed on covered surface 24 of substrate 10.Initial film layer 22 is heated at a temperature of about 20° C. toabout 120° C. for about 1 minute to about 24 hours, thereby formingstructure 30 comprising polyhemiaminal (PHA) film layer 32 disposed onthe covered surface 34 of substrate 10. PHA film layer 22 issubstantially free of solvent and/or water.

The substrate can be any suitable substrate, in particular any substratewhose Young's modulus is a factor of 5 greater than the polyhemiaminaland/or polyhexahydrotriazine. Non-limiting examples of these materialsinclude semiconductor wafers (e.g., silicon wafers), most metals,refractory materials, and possibly harder polymers.

The solvent mixture containing the PHA can be cast onto the substrateusing any suitable coating technique (e.g., spin coating, dip coating,roll coating, spray coating, and the like).

Also disclosed is a method of preparing a polyhexahydrotriazine (PHT)film from a PHA film, illustrated in the cross-sectional layer diagramsof FIG. 2. The polyhemiaminal film layer 32 of structure 30 can beheated at a temperature of at least 150° C., preferably about 165° C. toabout 280° C. even more preferably at about 180° C. to about 220° C.,and most preferably at about 200° C., thereby forming structure 40comprising polyhexahydrotriazine (PHT) film layer 42 disposed on coveredsurface 44 of substrate 10. The heating time at any of the abovetemperatures can be about 1 minute to about 24 hours. PHT film layer 42is substantially free of solvent and water. The hemiaminal groups of thePHA film are substantially or wholly converted to hexahydrotriazinegroups by heating the PHA film at a temperature in this range.

The number average molecular weight (Mn) of the PHA and/or PHT polymerscan be in a range of 1000 to 100,000, preferably in a range of 1000 to50,000, and most preferably in a range of 1000 to 20,000.

The polyhexahydrotriazines are attractive for applications requiringlightweight, rigid, strong thermosets such as aerospace engineering,electronics, and as mixtures for increasing the modulus of known resinsand composites.

The following examples illustrate the preparation of the PHA and PHTsolids and films, and the characterization of their physical properties.

EXAMPLES

Materials used in the following examples are listed in Table 1.

TABLE 1 ABBREVIATION DESCRIPTION SUPPLIER PF Paraformaldehyde SigmaAldrich PD p-Phenylenediamine Sigma Aldrich 4-Å molecular sieves SigmaAldrich DMF Dimethylformamide Sigma Aldrich PC Propylene carbonate SigmaAldrich NMP N-Methylpyrollidone Sigma Aldrich DPDN,N-dimethyl-p-phenylenediamine Sigma Aldrich HTPTHexahydro-1,3,5-tripheny1- Prepared below 1,3,5-triazine MDA4,4′-Methylenedianiline Sigma Aldrich ODA 4,4′-Oxydianiline SigmaAldrich FDA 4,4′-(9-fluorenylidene)dianiline, Sigma Aldrich MW 348.4PEG-DA Poly(ethylene glycol) diamine, Prepared belowH₂N(CH₂CH₂O)_(n)CH₂CH₂NH₂; Mn 4.6 kDa HDMA 1,6-Hexanediamine SigmaAldrich

Herein, Mn is the number average molecular weight, Mw is the weightaverage molecular weight, and MW is the molecular weight of onemolecule.

N-Methyl-2-pyrrolidone (NMP), paraformaldehyde,4,4′-diaminephenylmethane (MDA), and 4,4′-(9-fluorenylidene)dianiline(FDA) were purchased from Aldrich and used as received.4,4′-Oxydianiline (ODA) was purchased from Aldrich, rinsed with acetoneand dried in an Abderhalden drying pistol overnight prior to use.d₉-NMP, d₆-DMSO and CDCl₃ were purchased from Cambridge IsotopeLaboratories (CIL) and used as received. d₉-NMP, d₆-DMSO and CDCl₃ werepurchased from Cambridge Isotope Laboratories (CIL) and used asreceived.

PEG-DA was prepared according to the procedure of D. L. Elbert and J. A.Hubbell, “Conjugate Addition Reactions Combined with Free-RadicalCross-Linking for the Design of Materials for Tissue Engineering,”Biomacromolecules 2001, 2, 430-441.

¹H NMR spectra were recorded on a Bruker Avance 400 spectrometer (400MHz). Chemical shifts are reported in ppm from tetramethylsilane withthe solvent resonance as the internal standard (CDCl₃: delta 7.26 ppm;d₆-DMSO: delta 2.50 ppm; d₉-NMP: delta 3.23, 2.58, 1.80; d₆-acetone:delta 2.05 ppm).

¹³C NMR spectra were recorded on a Bruker Avance 400 spectrometer (100MHz) spectrometer with complete proton decoupling. Chemical shifts arereported in ppm from tetramethylsilane with the solvent resonance as theinternal standard (CDCl₃: delta 77.16 ppm; d₆-DMSO: delta 39.51). Dataare reported as follows: chemical shift, integration, multiplicity(s=singlet, d=doublet, t=triplet, q=quartet, sep=septet, bs=broadsinglet, m=multiplet), and coupling constants (Hz).

Infrared (IR) spectra were recorded on a Thermo Nicolet Nexus 670 FT-IRAlpha spectrophotometer using a Nicolet OMNI-Sampler ATRSmart-Accessory, with ν_(max) in cm⁻¹.

Gel permeation chromatography (GPC) was performed in THF or DMF using aWaters system equipped with four 5-micrometer Waters columns (300 mm×7.7mm) connected in series with an increasing pore size (100, 1000, 10⁵,10⁶ Å), a Waters 410 differential refractometer, and a 996 photodiodearray detector. The system was calibrated with polystyrene standards.

X-ray diffraction (XRD) profiles were obtained on Bruker D8 Discoverdiffractometer fitted with a 2D detector. All scans were performed in asymmetric theta-2theta geometry using graphite monochromated Cu-K_(α)x-rays (λ=1.5418 Å).

Atomic force microscopy (AFM) images were acquired on commercialinstrumentation under ambient conditions in intermittent contact mode(‘tapping’) at a 1 Hz scan rate with doped silicon cantilevers of springconstant of about 50 N/m.

Cross-polarization (CP) magic-angle-spinning (MAS) ¹³C NMR spectra wereobtained with a Bruker Avance 500 spectrometer operating at 125.762 MHzand 500.102 MHz for ¹³C and ¹H, respectively. Ramped-amplitude crosspolarization and two-pulse phase-modulated (TPPM) ¹H decoupling wereemployed. The amplitude of the ¹³C spin locking field was ramped from 80to 100% during the contact time and the ¹H decoupling field was 119 kHz(γB₁/2π). The MAS spinning speed was 14 kHz, the contact time was 3milliseconds, and the relaxation delay was 5 seconds. Spectra wereobtained by averaging 16000 scans on 23-30 mg of sample contained in a 4mm OD Bruker MAS rotor. The ¹³C chemical shifts were externallyreferenced using a rotor containing liquid tetramethylsilane (TMS).

Syntheses Example 1 (Comparative)

Reaction of aniline with paraformaldehyde to form hexahydrotriazinecompound 4,4′,4″-(1,3,5-triazinane-1,3,5-triyl)tris(N,N-dimethylaniline)(HTPT).

N,N-dimethyl-p-phenylenediamine (DPD, 0.21 g, 0.15 mmol) andparaformaldehyde (PF, 0.0046 g, 0.15 mmol, 1 equivalent (eq.)) wereweighed out into a 2-Dram vial inside a glovebox. DMSO (0.91 g, 1.0 mL)was added. The reaction mixture was removed from the glovebox, andheated in an oil bath at 180° C. for 20 minutes. The DMSO was removed invacuo and4,4′,4″-(1,3,5-triazinane-1,3,5-triyl)tris(N,N-dimethylaniline) wascollected as a brown solid (0.04 g, 79% yield).

The following procedure was used for a ¹H NMR time study of hemiaminalformation. DPD (0.021 g, 1.6 mmol (FIG. 3, ¹H NMR)) and PF (0.0057 g,1.9 mmol, 1.2 eq.) were carefully weighed into a dried 2-Dram vial withstirbar in the dry box and d₆-DMSO (1.0 mL, 1.6 M) was added by syringe.The mixture was transferred to a dried NMR tube and the condensationreaction was monitored over time. At 50° C. (FIG. 4, ¹H NMR), there aresignals corresponding to the formation of hemiaminal, and nohexahydrotriazine is observed. After heating at 180° C., however, >98%conversion to the hexahydrotriazine product HTPT is observed (FIG. 5, ¹HNMR).

The purified HTPT has a singlet resonating at delta 4.5 ppm (FIG. 6, ¹HNMR spectrum) for the six methylene protons of HTPT. ¹H NMR (d₆-DMSO,400 MHz): delta 6.97 (d, 2H, J=8 Hz), 6.66 (d, 2H, J=8 Hz), 4.53 (s,2H), 2.78 (s, 6H) ppm.

Example 2

Preparation of polyhemiaminal P-1 by reaction of 4,4′-oxydianiline (ODA)with paraformaldehyde (PF). The product is a powder.

4,4′-Oxydianiline (ODA, 0.20 g, 1.0 mmol) and paraformaldehyde (PF, 0.15g, 5.0 mmol, 5 equivalents (eq.)) were weighed out into a 2-Dram vialinside a N₂-filled glovebox. N-methylpyrrolidone (NMP, 6.2 g, 6.0 mL)was added (0.17 M). The vial was capped but not sealed. The reactionmixture was removed from the glovebox, and heated in an oil bath at 50°C. for 24 hours (after approximately 0.75 hours, the polymer begins toprecipitate in NMP). The polyhemiaminal P-1 was precipitated in acetoneor water, filtered and collected to yield 0.22 g, >98% yield as a whitesolid. ¹³C NMR (solid-state): 70, 120, and 145 ppm (FIG. 7).

Example 3

Preparation of polyhemiaminal P-2.

P-2 was prepared according to the preparation of P-1 (Example 1)substituting ODA with 4,4′-methylenedianiline (MDA) and using an MDA:PFmole ratio of 1:5. Isolated 0.15 g, 69% yield of an amorphous, insolubleoff-white powder. IR (KBr pellet) ν_(max): 3441 (br s), 2950 (w), 2931(w), 2885 (w), 1665 (s), 1507 (m), 1475 (w), 1427 (w), 1406 (w), 1301(m), 1264 (w), 1228 (w), 1115 (w), 1026 (w), 987 (w), 659 (w) cm⁻¹.

Example 4

Preparation of polyhemiaminal P-3.

P-3 was prepared according to the preparation of P-1 (Example 1)substituting ODA with 4,4′-(9-fluorenylidene)dianiline (FDA) and usingan FDA:PF mole ratio of 1:5. Isolated 0.26 g, 76% yield of an amorphous,insoluble white powder. IR (KBr pellet) ν_(max): 3442 (br s), 3063 (brw), 2955 (br w), 1659 (m), 1608 (m), 1502 (m), 1445 (w), 1384 (w), 1012(w), 814 (w), 741 (w) cm⁻¹.

Example 5

Preparation of polymer P-4.

P-4, a polyhexahydrotriazine, was prepared by reaction of4,4′-oxydianiline (ODA) with paraformaldehyde (PF). ODA (0.20 g, 1.0mmol) and PF (0.15 g, 5.0 mmol, 2.5 eq.) were weighed out into a 2-Dramvial inside a N₂-filled glovebox. NMP (6.2 g, 6.0 mL, 0.17 M) was added.The reaction mixture was removed from the glovebox, and heated in an oilbath at 200° C. for 3 hours (after approximately 0.25 hours, the polymerbegins to gel in the NMP). The solution was allowed to cool to roomtemperature and the polymer was precipitated in 40 mL of acetone,allowed to soak for 12 hours, then filtered and dried in a vacuum ovenovernight and collected to yield 0.21 g, 95% yield of P-4 as anoff-white solid.

Table 2 summarizes the above polyhemiaminal (PHA) andpolyhexahydrotriazine (PHT) preparations that were isolated as solids.

TABLE 2 Monomer PF Monomer:PF Reaction Polymer Example Name Name (g)(mmol) (g) (mmol) (mole ratio) Solvent Conditions Type^(a) Description 2P-1 ODA 0.200 1.0 0.150 5.0 1:5 NMP 50° C., 24 hrs PHA white solid 3 P-2MDA 0.198 1.0 0.150 5.0 1:5 NMP 50° C., 24 hrs PHA off-white solid 4 P-3FDA 0.348 1.0 0.150 5.0 1:5 NMP 50° C., 24 hrs PHA white solid 5 P-4 ODA0.200 1.0 0.150 5.0 1:5 NMP 200° C., 3 hrs PHT off-white solid ^(a)PHA =polyhemiaminal, PHT = polyhexahydrotriazine

Film Preparations Polyhemiaminal (PHA) Films Example 6

Preparation of polyhemiaminal film PHA-1. 4,4′-Oxydianiline (ODA, 0.400g, 2.0 mmol) and paraformaldehyde (PF, 0.300 g, 10.0 mmol, 5 eq.) wereweighed into a 2-Dram vial with equipped with a stirbar. NMP (6 mL, 0.33M with respect to ODA) was added to the vial under nitrogen. The vialwas capped but not sealed. The solution was stirred at 50° C. for 30minutes (time sufficient to form soluble oligomers in NMP). The clearand colorless solution was then filtered through a nylon syringe filter(0.45 micrometers) onto a glass plate with aluminum tape (80 micrometersthickness) boundaries. The film was cured at 50° C. for 24 hours. Theclear and colorless polyhemiaminal film was then carefully peeled fromthe glass plate using a razor blade. IR (film), ν_(max) (cm⁻¹): 3389 (brs), 3031 (m), 2871 (m), 1863 (w), 1667 (s), 1609 (s), 1471 (s), 1404(m), 1304 (m), 1297 (s), 1128 (m), 1078 (m), 1007 (m), 871 (m), 821 (s),511 (m).

Example 7

Preparation of polyhemiaminal film PHA-2. PHA-2 was prepared accordingto Example 6 using an ODA:PF mole ratio of 1:6.7.

Example 8

Preparation of polyhemiaminal film PHA-3. PHA-3 was prepared accordingto Example 6 using an ODA:PF mole ratio of 1:10.

Example 9

Preparation of polyhemiaminal film PHA-4. PHA-4 was prepared accordingto Example 6, substituting ODA with 4,4′-methylenedianiline (MDA) andusing an MDA:PF mole ratio of 1:5.

Example 10

Preparation of polyhemiaminal film PHA-5. PHA-5 was prepared accordingto Example 6, substituting ODA with 4,4′-fluorenylidenedianiline (FDA)and using an FDA:PF mole ratio of 1:5.

Example 11

Attempted preparation of polyhemiaminal film PHA-6.

PHA-6 was prepared according to Example 6, substituting ODA with1,6-hexanediamine (HDMA) and using an HDMA:PF mole ratio of 1:5. Acontinuous, regular film was not obtained. Instead small pieces ofpolymer were removed from the glass with a razor blade.

Example 12

Preparation of polyhemiaminal film PHA-7.

PHA-7 was prepared according to Example 6, substituting ODA withpoly(ethylene glycol) diamine (PEG-DA) and using an PEG-DA:PF mole ratioof 1:5.

Polyhexahydrotriazine (PHT) Films Example 13

Preparation of polyhexahydrotriazine film PHT-1. ODA (0.400 g, 2.0 mmol)and PF (0.150 g, 5.0 mmol, 2.5 equiv) were weighed into a 2-Dram vialequipped with a stirbar. NMP (6 mL, 0.33 M with respect to ODA) wasadded to the vial under nitrogen and the vial was capped. The vial wasnot sealed. The solution was allowed to stir at 50° C. for 30 minutes(time sufficient for solubility of reagents in NMP). The clear andcolorless solution was then filtered through a nylon syringe filter(0.45 micrometer) onto a leveled glass plate with aluminum tape (80micrometers thickness) boundaries and allowed to cure according to thefollowing ramping procedure: 22° C. to 50° C. over 1 hour; then 50° C.to 200° C. over 1 hour, and hold at 200° C. for 1 hour. The yellow filmwas then carefully peeled from the glass plate using a razor blade. IR(film), ν_(max) (cm⁻¹): 3042 (w), 2815 (w), 1679 (w), 1495 (s), 1385(w), 1211 (s), 1111 (w), 985 (w), 936 (w), 871 (w), 826 (w).

Example 14

Preparation of polyhexahydrotriazine film PHT-2. PHT-2 was preparedaccording to Example 13 using an ODA:PF mole ratio of 1:5.

Example 15

Preparation of polyhexahydrotriazine film PHT-3. PHT-3 was preparedaccording to Example 13 using an ODA:PF mole ratio of 1:10.

Example 16

Preparation of polyhexahydrotriazine film PHT-4. PHT-4 was preparedaccording to Example 13, substituting NMP with propylene carbonate (PC).The film adhered strongly to the glass plate. Attempted separation ofthe film using a razor blade produced powder scrapings.

Example 17

Preparation of polyhexahydrotriazine film PHT-5. PHT-5 was preparedaccording to Example 13, substituting ODA with 4,4′-methylenedianiline(MDA) and using an MDA:PF mole ratio of 1:2.5.

Example 18

Preparation of polyhexahydrotriazine film PHT-5. PHT-5 was preparedaccording to Example 13, substituting ODA with4,4′-fluorenylidenedianiline (FDA) and using an FDA:PF mole ratio of1:2.5.

Example 19

Attempted preparation of polyhexahydrotriazine film PHT-6. PHT-6 wasprepared according to Example 13, substituting ODA with1,6-hexanediamine (HDMA) and using an HDMA:PF mole ratio of 1:2.5. Nocontinuous film was formed. Small pieces of polymer were removed fromthe glass plate with a razor blade.

Examples 20

Attempted preparation of polyhexahydrotriazine film PHT-7. PHT-7 wasprepared according to Example 13, substituting ODA with poly(ethyleneglycol) diamine (PEG-DA) and using an PEG-DA:PF mole ratio of 1:2.5. Itappeared as though some of the PEG-DA had remained on the glass melted.No PHT or film was obtained.

Table 3 summarizes the above polyhemiaminal (PHA) andpolyhexahydrotriazine (PHT) film preparations.

TABLE 3 Monomer PF Monomer:PF Cure Example Name Name (g) (mmol) (g)(mmol) (mole ratio) Solvent Conditions Description 6 PHA-1 ODA 0.200 1.00.150 5.0 1:5  NMP 50° C., 24 hrs Clear and colorless 7 PHA-2 ODA 0.2001.0 0.225 7.5 1:6.7 NMP 50° C., 24 hrs Clear and colorless 8 PHA-3 ODA0.200 1.0 0.300 10 1:10  NMP 50° C., 24 hrs Clear and colorless 9 PHA-4MDA 0.198 1.0 0.150 5.0 1:5  NMP 50° C., 24 hrs Clear and light yellow10 PHA-5 FDA 0.348 1.0 0.150 5.0 1:5  NMP 50° C., 24 hrs Clear andcolorless 11 PHA-6 HMDA 0.116 1.0 0.150 5.0 1:5  NMP 50° C., 24 hrs nofilm 12 PHA-7 PEG-DA 0.460 0.1 0.015 0.5 1:5  NMP 50° C., 24 hrs Cloudyand colorless, elastic 13 PHT-1 ODA 0.400 2.0 0.300 5.0 1:2.5 NMP 50°C., 1 hr; Clear and light ramp 50-200° yellow/orange C. over 1 hr; 200°C. 1 hr 14 PHT-2 ODA 0.400 2.0 0.450 10.0 1:5  NMP 50° C., 1 hr; Clearand ramp 50-200° yellow/orange C. over 1 hr; 200° C. 1 hr 15 PHT-3 ODA0.400 2.0 0.600 20.0 1:10  NMP 50° C., 1 hr; Clear and dark ramp 50-200°orange, with C. over 1 hr; some lighter 200° C. 1 hr patches 16 PHT-4ODA 0.200 1.0 0.75 2.5 1:2.5 PC 50° C., 1 hr; Brown and ramp 50-200°brittle C. over 1 hr; 200° C. 1 hr 17 PHT-5 MDA 0.198 1.0 0.075 2.51:2.5 NMP 50° C., 1 hr; Clear and ramp 50-200° yellow film C. over 1 hr;200° C. 1 hr 18 PHT-6 FDA 0.348 1.0 0.075 2.5 1:2.5 NMP 50° C., 1 hr;Clear and ramp 50-200° colorless film, C. over 1 hr; somewhat 200° C. 1hr brittle 19 PHT-7 HMDA 0.116 1.0 0.075 2.5 1:2.5 NMP 50° C., 1 hr; nofilm ramp 50-200° C. over 1 hr; 200° C. 1 hr 20 PHT-8 PEG-DA 0.460 0.10.0075 0.25 1:2.5 NMP 50° C., 1 hr; no film ramp 50-200° C. over 1 hr;200° C. 1 hr

Table 4 summarizes the swelling characteristics of the PHA and PHT filmsdetermined in NMP, aqueous sulfuric acid solution, saturated sodiumbicarbonate solution, and 35% aqueous hydrogen peroxide solution.

TABLE 4 % Swelling of Film In NMP and Aqueous Solutions 0.5M Monomer:PFCure Film H₂SO₄ NaHCO₃ 35% Example Film Monomer (mole ratio)Conditions^(a) Forming? NMP (pH 0) (pH 11) H₂O₂ 6 PHA-1 ODA 1:5  1 Yes 1decomp −2.00 decomp 7 PHA-2 ODA 1:6.7 1 Yes −10 decomp −0.20 decomp 8PHA-3 ODA 1:10  1 Yes 8 decomp −0.90 decomp 9 PHA-4 MDA 1:5  1 Yes 10PHA-5 FDA 1:5  1 Yes 11 PHA-6 HMDA 1:5  1 No 12 PHA-7 PEG-DA 1:5  1 YesFilm dissolves in water 13 PHT-1 ODA 1:2.5 2 Yes 0.03 decomp −0.50 0.4014 PHT-2 ODA 1:5  2 Yes 1.9 −56 2.40 9.20 15 PHT-3 ODA 1:10  2 Yes 0.20decomp −3.20 −0.40 16 PHT-4 ODA 1:2.5 3 Yes decomp 17 PHT-5 MDA 1:2.5 2Yes 18 PHT-6 FDA 1:2.5 2 Yes 19 PHT-7 HMDA 1:2.5 2 No 20 PHT-8 PEG-DA1:2.5 2 No ^(a)1 = solvent was NMP, 50° C. for 24 hours; 2 = solvent wasNMP, 50° C. for 1 hour; ramp 50-200° C. over 1 hour, 200° C. for 1 hour;3 = solvent was PC, 50° C. for 1 hour, ramp 50-200° C. over 1 hour, 200°C. for 1 hour.

Acid Treatment of Polyhexahydrotriazine Film and ODA Recovery

A 0.050-g sample of PHT-1 film was exposed to 20-mL of 0.5 M H₂SO₄(pH=0) for 36 hours until complete disintegration had occurred. Thesolution was neutralized with sodium bicarbonate until the pH was deemedto be approximately 7.0 (by testing with pH strip). A precipitate formedin the solution, which was isolated by filtration to yield 40.0 mg ofrecovered ODA starting material as a white powder. ¹H NMR spectra matchpreviously reported values. Notably, a 15.7-mg sample in a saturatedNH₄Cl pH=5.5 solution showed no decomposition after 24 hours.

Thermal Analysis of Films Cured at Various Temperatures.

Table 5 summarizes the Tg, storage modulus, and temperature for 5% massloss of the films formed. Tg was measured using dynamic mechanicalanalysis (DMA) and/or differential scanning calorimetry (DSC) followingtreatment of the sample using one of two procedures A and/or B. Inprocedure A, data was collected on a sample heated to 50° C. andanalyzed directly. In procedure B, data was collected after holding thesample at 200° C. or one hour. Temperature for 5% mass loss wasdetermined by thermogravimetric analysis (TGA) following treatment ofthe sample using procedures A or B. For examples that did not formcontinuous films, the Tg was measured on pieces of polymer obtained fromscrapings of the polymer from the glass plate using a razor blade.

TABLE 5 Storage Tg by Tg by Modulus Temp for Temp for Monomer:PF CureFilm DMA DSC by DMA 5% mass 5% mass Example Film Monomer (mole ratio)Conditions^(a) Forming? (° C.)^(b) (° C.)^(c) (GPa) loss (° C.)^(d) loss(° C.)^(e) 6 PHA-1 ODA 1:5  1 Yes 115 125   0.1 165 238 (A) (A) 222.27(B) 7 PHA-2 ODA 1:6.7 1 Yes 100 220.12 0.2 200 294 (A) (B) 8 PHA-3 ODA1:10  1 Yes 210  73.43 0.75 150 284 (A) (B) 9 PHA-4 MDA 1:5  1 Yes 120125.6  2.2 165.5 (A) (A) 10 PHA-5 FDA 1:5  1 Yes 75  112.5 (A) 11 PHA-6HMDA 1:5  1 No 173.28 167 (A) 12 PHA-7 PEG-DA 1:5  1 Yes  49.04 0.7 350(A) 13 PHT-1 ODA 1:2.5 2 Yes 192 193   3.8 238 (A) (A) 14 PHT-2 ODA 1:5 2 Yes   207.2 217.58 3.25 275 (A) (A) 15 PHT-3 ODA 1:10  2 Yes 218200.35 0.9 238 (A) (A) 16 PHT-4 ODA 1:2.5 3 Yes 146.95 180 (A) 17 PHT-5MDA 1:2.5 2 Yes 227 2.4 250 (A) 18 PHT-6 FDA 1:2.5 2 Yes   229.17 3.0150 (A) 19 PHT-7 HMDA 1:2.5 2 No  55.51 150 (A) 20 PHT-8 PEG-DA 1:2.5 2No ^(a)1 = solvent was NMP, 50° C. for 24 hours; 2 = solvent was NMP,50° C. for 1 hour; ramp 50-200° C. over 1 hour, 200° C. for 1 hour; 3 =solvent was PC, 50° C. for 1 hour, ramp 50-200° C. over 1 hour, 200° C.for 1 hour. ^(b)Tg = glass transition temperature obtained by dynamicmechanical analysis (DMA); Codes A and B represent the following: A =data collected on a run heated to 50° C and analyzed directly, B = dataobtained after holding sample at 200° C. or one hour ^(c)Tg obtained bydifferential scanning calorimetry (DSC); Codes A and B represent thefollowing: A = data collected on a run heated to 50° C. and analyzeddirectly, B = data obtained after holding sample at 200° C. or one hour^(d)Determined by thermogravimetric analysis (TGA); data collected on arun heated to 50° C. and analyzed directly ^(e)Determined bythermogravimetric analysis (TGA); data obtained after holding sample at200° C. or one hour

The glass transition temperatures (Tg) of polyhemiaminal (PHA) filmsgiven different thermal treatments were evaluated through DMA and DSC.In the first scan of PHA-1 (Example 6) cured at 50° C. with nosubsequent higher heat treatment, the observed Tg was about 125° C. byDSC (FIG. 8, graph), which is notably higher than the cure temperature(50° C.). When the sample was cured at 50° C., then heated at 200° C.for one hour prior to analysis, the Tg increased to 222.27° C. by DSC(FIG. 9, graph). The increase in Tg is likely due to a chemicaltransformation taking place at higher temperature (i.e., the transitionfrom polyhemiaminal to polyhexahydrotriazine).

The first DSC scan of the PHT-1 film (Example 13) is shown in FIG. 10.The Tg (192° C.) is close to the curing temperature (200° C.).

Determination of Mass Lost During Polyhemiaminal Heating

A colorless polyhemiaminal film (0.138 g) formed with 4,4′-oxydianiline(ODA) and cured at 50° C. was placed in a capped 5-Dram glass vial underN₂ and sealed with Teflon tape. The vial was submerged halfway into asand bath heated at 200° C. for 24 hours. The vial was then removed fromthe heat, allowed to cool to room temperature, and brought into anitrogen filled glovebox. Soluble condensates that formed on the upperportion of the vial were rinsed with 2-5 mL portions of dry d₆-acetone(<5% H₂O by ¹H NMR analysis after drying over 4 Å molecular sieves for24 hours), transferred to an oven-dried NMR tube, and 1 microliter ofanhydrous benzene as an internal standard was added to the NMR tube. Theremaining brown film was removed from the vial and weighed (0.0657 g,lost 52% of original mass). Yields determined by ¹H NMR analysis of NMPand water were 0.0271 g (0.273 mmol) and 0.0106 g (0.588 mmol)respectively, accounting for 52% of the total mass lost during heating.The remaining 48% of the film mass loss was assigned to ODA-basedoligomers that sublime at 200° C. No monomeric ODA was observed afterheating, indicating that the initial polyhemiaminal had formed to >98%conversion.

FIG. 11 is a graph showing the dynamic mechanical analysis (DMA) andthermogravimetric analysis (TGA) of ODA films under varying film cureconditions. Films cured at 50° C. (second curve from top) exhibit alarger mass loss prior to their decomposition temperature compared tothose cured at 200° C. (top curve). This discrepancy is explained byloss of NMP and water when converting the polyhemiaminal (PHA) to thepolyhexahydrotriazine (PHT) at elevated temperature. The DMA profile(bottom curve) following a reaction between ODA and paraformaldehydeshows two thermal transitions in the first heat ramp, likelycorresponding to reaction to form the PHA, followed by loss of NMP andwater and concurrent ring closure to form the PHT. The second heat ramp(second from bottom curve) shows a storage modulus characteristic of ahigh-modulus polymer, with a shallow transition and only slight variancein modulus over the temperature range. The Tg observed at about 275° C.in the second heat ramp corresponds to an ODA-based PHT film cured at200° C.

Atomic Force Microscopy (AFM) Images of PHA and PHT Films

AFM images were taken of a polyhemiaminal film PHA-1 (Example 6) curedat 50° C. (FIG. 12) and polyhexahydrotriazine film PHT-1 (Example 13)cured at 200° C. (FIG. 13) cast on silicon wafers. In each case, thesurface of the film was smooth with a root mean square (RMS) roughnessof 3-4 Å, consistent with an amorphous, highly crosslinked polymericstructure. The PHA-1 film showed slightly higher granularity compared tothe PHT-1 film. The PHT-1 film exhibited evenly distributed holesthroughout the polymer, with a depth of 22 nm and width of 55 nm, likelydue to the NMP and water removal from the film at high curingtemperature.

¹H NMR Analysis of Soluble Polymer Networks

In an attempt to form a soluble polymer for ¹H NMR analysis,4,4′-(9-fluorenylidene)dianiline (FDA) was used as a monomer (FIG. 14,¹H NMR of FDA) in place of ODA for polymerization. Under diluteconditions, the FDA-based polymers obtained after both 50° C. treatmentfor 0.5 hour and 200° C. treatment for 0.5 hour were sufficientlysoluble in d₆-DMSO such that the polymer could be analyzed. Sharpsignals, likely corresponding to the N—H, and O—H groups in thepolyhemiaminal (PHA) were present in the spectrum of the samplepolymerized at 50° C. for 2 hours (FIG. 15, ¹H NMR spectrum labeled A)whereas the spectrum of the polyhexahydrotriazine (PHT) sample formed at200° C. for 2 hours (FIG. 15, ¹H NMR spectrum labeled B) showed onebroad signal in the delta 4-5 ppm region.

The PHT film formed from FDA was not found to be soluble in d₆-DMSO (no¹H NMR signals corresponding to PHA, PHT, or starting dianiline wereobserved when the film was submerged in d₆-DMSO in an NMR tube),indicating that removal of the solvent during polymerization andconcentration of the solution was necessary for high crosslinking whenusing FDA as a monomer.

Gel Permeation Chromatography (GPC)

FIG. 16 is a GPC trace using THF as eluent of the product formed byreaction of FDA with 2.5 equivalents of paraformaldhyde for 2 hours at50° C. in 0.33 M NMP. The polyhemiaminal was purified by precipitationin water. Mw=10,576, Mn=3,588, and PDI=2.95.

FIG. 17 is a GPC trace using THF as eluent of the product formed by thereaction of FDA with 2.5 equivalents of paraformaldhyde for 2 hours at200° C. in 0.33 M NMP. Mw=48,910; Mn=8,174; PDI=5.9. The 200° C.treatment resulted in an Mn value about two times that of thepolyhemiaminal, indicating that chain growth occurs at the highertemperature in addition to conversion of the hemiaminal tohexahydrotriazine.

Effect of Water and Formic Acid on Hexahydrotriazine Formation

A slight increase in conversion was observed when water was added to apolymerization solution of ODA and paraformaldehyde in DMSO at 50° C.(91% conversion vs 94% conversion, as shown in Table 6 below, entries 1and 2). Addition of 50 mol % formic acid to the reaction mixturedecreased the conversion to 50% (Table 6, entry 4). Also, the reactionswith formic acid did not form gels in solution, whereas those with waterdid form gels. For this series of experiments, DMSO was refluxed for 108hours over CaH₂ and distilled prior to use. Paraformaldehyde was washedwith saturated NaHCO₃ to remove any acid impurities and dried prior touse. Percent conversion was determined by the ¹H NMR determination ofnew compounds with aromatic protons within 10 minutes of heating at 50°C.

TABLE 6 Entry Conditions % Conversion 1 No additive- dry DMSO solvent 912 50 mol % water added 94 3 10 mol % water added 90 4 50 mol % formicacid 50 5 10 mol % formic acid 65

X-Ray Diffraction (XRD) Analysis of ODA-Based PHT Films

FIG. 18 is an XRD pattern observed for crystalline ODA monomer. FIG. 19is an XRD pattern showing diffuse scattering observed for an ODA basedPHT film.

Young's Modulus and Hardness Obtained by Nanoindentation of Films

Nanoindentation measurements were performed using a HysitronTriboindenter (TI-950) with a cube-corner probe (FIG. 20, drawing)having a pyramid angle of 90° and an end radius of about 50 nm. Anglea=90° C. in FIG. 20. The sample films had a thickness of about 4micrometers and were mounted with a cyanoacrylate (SUPER GLUE) onto astainless steel substrate 15 mm in diameter and 700 micrometers thick.The thickness of the films (about 4 micrometers) was much greatercompared to the indentation depth (about 100 nm). Therefore, substrateeffects were minimal if not absent from the measurements. The loadfunction was 5-2-5 (i.e., 5 second load, 2 second hold, and 5 secondunload). Prior to indenting onto samples of interest, the probe wascalibrated using a quartz sample having a thickness of 1 mm, which wasmounted using the same procedure as the samples. A total of threecoefficients (for the zeroth, first and second order, respectively) wereused to calibrate the probe, as the indentation depth was small. Thequartz modulus was 70 GPa and hardness was 9.7 GPa. Between the samplesthe probe was both cleaned and calibrated using indents in quartz. Sevendifferent indentation loads were chosen for measurements on each sample:3 μN, 5 μN, 7.5 μN, 10 μN, 12.5 μN, 15 μN, and 20 μN. Seven independentrepeats of each load separated at least by 10 micrometers in the X andthe Y dimensions were performed to avoid any possibility of strain fieldcoupling between two indents. For each load, the modulus was measuredand an average modulus was determined from at least 25 independent datapoints from between 5 μN and 15 μN. The raw data was corrected for thepoint of zero force and zero displacement. This is a standard offsetmade for low load indents and lends to more correct use ofnanoindentation models.

The reported average reduced modulus values are at least 20 independentdata points averaged together. The graphs were plotted with +/− onestandard deviation on the values for each load.

FIG. 21 is a graph showing the reduced modulus and hardness data forExample 6.

FIG. 22 is a graph showing the reduced modulus and hardness data forExample 13.

FIG. 23 is a graph showing the reduced modulus and hardness data forExample 14.

FIG. 24 is a graph showing the reduced modulus and hardness data forExample 15.

Table 7 summarizes the reduced modulus (Er) and hardness (H) of thefilms determined by nanoindentation. Example 6 is a polyhemiaminal film,and Examples 13-15 are polyhexahydrotriazine films.

TABLE 7 Cure Monomer:PF Condition Film Er H Example Name Monomer (moleratio) (see Code)^(a) Forming? (GPa)^(b) (GPa)^(b) 6 PHA-1 ODA 1:5  1Yes 6.3 0.86 7 PHA-2 ODA 1:6.7 1 Yes 8 PHA-3 ODA 1:10  1 Yes 9 PHA-4 MDA1:5  1 Yes 10 PHA-5 FDA 1:5  1 Yes 11 PHA-6 HMDA 1:5  1 No 12 PHA-7PEG-DA 1:5  1 Yes 13 PHT-1 ODA 1:2.5 2 Yes 14 2 14 PHT-2 ODA 1:5  2 Yes8 1.9 15 PHT-3 ODA 1:10  2 Yes 11 1.9 16 PHT-4 ODA 1:2.5 3 Yes 17 PHT-5MDA 1:2.5 2 Yes 18 PHT-6 FDA 1:2.5 2 Yes 19 PHT-7 HMDA 1:2.5 2 No 20PHT-8 PEG-DA 1:2.5 2 No ^(a)Codes 1, 2, and 3 represent the followingreaction conditions: 1 = NMP, 50° C., 24 hours; 2 = NMP, 50° C., 1 hour,ramp 50-200° C. over 1 hour, 200° C. 1 hour; 3 = PC, 50° C., 1 hour,ramp 50-200° C. over 1 hour, 200° C. 1 hour; ^(b)Er = reduced modulus; H= hardness determined by nanoindentation

Without being bound by theory, the 6 GPa spread in Examples 13-15 isbelieved to be due to variability in film uniformity as theparaformaldehyde content was increased from 2.5 equivalents to 10equivalents. As the paraformaldehyde level was increased, holes formedwhere the formaldehyde blew out of the film during casting. Examples 14and 15 were non-uniform, whereas Example 13 was highly uniform. Residuallevels of excess paraformaldehyde can also plasticize the film, loweringthe modulus significantly.

Density Functional Theory (DFT) Calculations

The dispersion-corrected B3LYP DFT method was used to investigate themechanism and energetics for 1,3,5-trimethyl-1,3,5-triazinane formationfrom the reaction of formaldehyde with methylamine, a simple(computationally inexpensive) amine, as shown in the reaction diagram ofFIG. 25).

The DFT calculations assume paraformaldehyde initially degrades in thereaction conditions to form a formaldehyde monomer that reacts with theamine to form N,N′-dimethylmethanimidamide:

Overall, the reaction proceeds through the consecutive addition of amineand formaldehyde reactants. The proposed mechanism invokes catalysis bytwo explicit water molecules throughout the reaction. A catalytic amountof water can be available to promote the reaction by way of traceamounts of water initially present in the reaction mixture and/or waterformed by the reaction of amine with formaldehyde.

The DFT calculations indicate that three general types oftransformations are involved in the reaction mechanism promoted bywater: (i) addition of formaldehyde to an amine, (ii) the formation ofimine by elimination of water byproduct, and (iii) addition of an amineto an imine (FIG. 25). Intermediates are smoothly transformed fromweakly hydrogen-bonded complexes to covalently bonded intermediateswithout discernible transition structures for transformations involvingcondensation of an amine with formaldehyde or with an imine. Transitionstructures could be found, however, for processes in which only protonsare shuttled from the amine to formaldehyde (or imine intermediates)with water molecules after the addition of an amine to formaldehyde orto an imine. These transformations possess low barriers, often less than3 kcal/mol. By comparison, transition structures for imineformation/water elimination could be found that possess much larger freeenergies of activation.

The largest calculated free energy barrier arises from elimination ofwater from INT18 (where “INT” refers to intermediate) in TS8 (where “TS”refers to transition state) (FIG. 26, energy diagram). While water lossduring previous stages in the mechanism, such as from INT3 in TS3 andfrom INT10 in TS6, results in the formation of a neutral imine, such atransformation from INT 18 would result in the formation of animinium-amide intermediate, which is unstable because of itszwitterionic character and is therefore responsible for this largefree-energy barrier. In fact, as confirmed by intrinsic reactioncoordinate (IRC) calculations, elimination of water from INT18 in TS8 isfollowed by spontaneous cyclization of the iminium-amide intermediate toform the 1,3,5-hexahydro-1,3,5-triazine in INT19 (FIG. 26). The largefree energy barrier between INT18 and TS9 is particularly importantbecause INT18 is the hemiaminal structure. The DFT calculations indicatethat the hemiaminal is a relatively stable intermediate during1,3,5-hexahydro-1,3,5-triazine formation.

The calculations are consistent with the ¹H NMR and IR analyses ofpolyhemiaminal and polyhexahydrotriazine formation. When formed witharomatic amines, the polyhemiaminal structure is experimentally observedto form at low temperatures (˜50° C.), and is a stable intermediate upto temperatures less than 150° C. Without being bound by theory, theformation of PHTs from aromatic diamines may require temperatures of atleast 150° C. in order to overcome energy barriers associated withsolvent and/or water stabilization of the polyhemiaminal structure.

DFT calculations indicate that the hemiaminal is further stabilized byNMP. Calculations revealed that displacement of bound water moleculesfrom INT18 by NMP solvent produces a complex in which NMP ishydrogen-bonded to the hexahydrotriazine precursor; this process isexothermic by 7.7 kcal/mol (FIG. 26). Thus, the total free energyrequired for hexahydrotriazine formation after exposure to NMP must be acombination of the energy required to displace the solvent and thatrequired to eliminate water, which is 34.9 kcal/mol. The DFTcalculations suggest that the condensation between ODA and formaldehydeat 50° C. results in the formation of a solvent-stabilized intractablepolyhemiaminal intermediate (i.e., the NMP/water-bound polyhemiaminal),whereas polyhexahydrotriazine formation will only occur at highertemperatures (e.g., 200° C.) where the water-bound polyhemiaminal isconverted to the polyhexahydrotriazine through a high-energy transitionstate.

The DFT calculations suggest that, when heated above the boiling pointof NMP and H₂O, a polyhemiaminal film will lose significant weightassociated with the loss of both, as they are tightly bound to thepolyhemiaminal at room temperature. In fact, thermogravimetric analysis(TGA) of ODA polyhemiaminal films cured at 50° C. showed 10-20% weightloss when heated above 100° C. (FIG. 11). Experiments also revealed that52% of the total mass lost while heating an ODA-based polyhemiaminalfilm was due to NMP and water desorption (thereby accounting for 27 wt %(weight percent) of the polyhemiaminal). Additionally, NMP swellingmeasurements used to survey the crosslink density of polyhemiaminalfilms showed no swelling. DFT calculations suggest that NMP stabilizesthe hemiaminal structure by 7.7 kcal/mol. No additional NMP uptake bythe film was observed, indicating NMP saturation is achieved duringpolyhemiaminal formation in the presence of excess NMP. TGA analysis upto ˜300° C. (the decomposition temperatures of the polymers) of ODAfilms cured at 200° C. showed less weight loss than the films cured at50° C., confirming that solvent is no longer present in the PHT filmafter high temperature curing, as is also indicated by the IRspectroscopy results.

CONCLUSION

The disclosed method of preparing polyhexahydrotriazines from electronrich aromatic diamines provides intermediate polyhemiaminals, which arestable up to temperatures less than 150° C. When heated to temperaturesof at least 150° C., the polyhemiaminals form crosslinkedpolyhexahydrotriazines having a high Tg and a high Young's modulus. Thepolyhexahydrotriazines are rigid according to DMA analysis.

Electron-deficient diamines did not form polyhemiaminals and weregenerally unreactive towards paraformaldehyde. Polyhemiaminal formationwas favored by electron-rich aromatic diamines. Optimal conditions forthe formation of polyhemiaminals used about 5 molar equivalents offormaldehyde relative to aromatic diamine. Decreasing the amount offormaldehyde resulted in polyhemiaminal films generally not suitable formechanical analysis, or produced low quality films. Increasing theamount of formaldehyde to 6.7 or 10 equivalents adversely affected themechanical properties of the films. For example, the PHA film Tgdecreased with increasing formaldehyde (e.g., Tg=115° C. for PHA-1 vsTg=100° C. for PHA-2). Also, a PHA film synthesized with 10 equivalentsof formaldehyde had a lower temperature for 5% mass loss compared to aPHA film formed with 5 equivalents formaldehyde (compare PHA-3 to PHA-1for 5% mass loss, 150° C. vs 165° C., Table 5), presumably due to thedesorption of the excess formaldehyde from the film. When the curingprocedure was adjusted for forming the polyhexahydrotriazine (PHT),films of high quality and excellent mechanical properties were obtainedusing 2-2.5 equivalents of formaldehyde.

PHA and PHT films formed with electron-rich bisanilines MDA and FDAgenerally did not perform as well as the ODA-based films. However, thesefilms still had high Tg and storage moduli as determined by DMA. Thepolymer prepared from hexamethylenediamine (HMDA) was not a goodfilm-former, and the polymer exhibited a lower Tg than aromatic basedPHAs and PHTs. The film formed with PEG-DA was found to have decreasedstorage modulus compared to its aromatic analogues as determined by DMA(700 MPa), and the PEG-DA based film dissolved in water upon standingfor 24 hours. When the temperature was increased to match conditions forPHT formation with anilines, no reaction to form a PHA film or PHT filmwas observed with PEG-DA.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. When a range is used to express apossible value using two numerical limits X and Y (e.g., a concentrationof X ppm to Y ppm), unless otherwise stated the value can be X, Y, orany number between X and Y.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiments were chosen and described in order to best explain theprinciples of the invention and their practical application, and toenable others of ordinary skill in the art to understand the invention.

What is claimed is:
 1. A polyhexahydrotriazine (PHT), comprising: aplurality of trivalent hexahydrotriazine groups having the structure

and a plurality of divalent bridging groups of formula (2):

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ comprises at least 1 carbon and R″comprises at least one carbon, each starred bond of a givenhexahydrotriazine group is covalently linked to a respective one of thedivalent bridging groups, and each starred bond of a given bridginggroup is linked to a respective one of the hexahydrotriazine groups. 2.The PHT of claim 1, wherein the PHT has a number average molecularweight (Mn) of about 1000 to about
 20000. 3. The PHT of claim 1, whereinL′ is *—O—*.
 4. The PHT of claim 1, wherein L′ is *—S—*.
 5. The PHT ofclaim 1, wherein L′ is *—N(R′)—*, wherein R′ is selected from the groupconsisting of methyl, ethyl, propyl, isopropyl, phenyl, and combinationsthereof.
 6. The PHT of claim 1, wherein L′ is *—CH2—*.
 7. The PHT ofclaim 1, wherein L′ is isopropylidenyl (*—C(Me₂)-*.
 8. The PHT of claim1, wherein L′ is 9-fluorenylidenyl:


9. The PHT of claim 1, wherein the PHT further comprises a monovalentaromatic group (diluent group) selected from the group consisting of:

and combinations thereof, wherein W′ is a monovalent radical selectedfrom the group consisting of *—N(R¹)(R²), *—OR³, —SR⁴, wherein R¹, R²,R³, and R⁴ are monovalent radicals independently comprising at least 1carbon, and the starred bond in each of formulas (8), (9), (10) and (11)is linked to a nitrogen of a hexahydrotriazine group of the PHT.
 10. ThePHT of claim 9, wherein the diluent group is:


11. A method, comprising: forming a reaction mixture comprising a i)solvent, ii) paraformaldehyde, and iii) a monomer comprising two primaryaromatic amine groups; and heating the reaction mixture at a temperatureof 150° C. to about 280° C., thereby forming a polyhexahydrotriazine(PHT).
 12. A method, comprising: forming a first mixture comprising a i)solvent, ii) paraformaldehyde, and iii) a monomer comprising two primaryaromatic amine groups; heating the first mixture at a temperature ofabout 20° C. to about 120° C., thereby forming a second mixturecomprising a polyhemiaminal (PHA); casting the second mixture on asurface of a substrate, thereby forming an initial film layer disposedon the surface, the initial film layer comprising the PHA and thesolvent; and heating the initial film layer at a temperature of 150° C.to about 280° C., thereby forming a second film layer comprising apolyhexahydrotriazine (PHT).
 13. The method of claim 12, wherein thesolvent is selected from the group consisting of N-methylpyrrolidone(NMP), propylene carbonate (PC), dimethylacetamide (DMA),dimethylsulfoxide (DMSO), propylen glycol methyl ether acetate (PGMEA),dimethylformamide (DMF), and combinations thereof.
 14. The method ofclaim 12, wherein the monomer is selected from the group consisting of4,4′-oxydianiline (ODA), 4,4′-methylenedianiline (MDA),p-phenylenediamine (PD), 4,4′-(9-fluorenylidenyl)dianiline (FDA), andcombinations thereof.
 15. The method of claim 12, wherein said heatingthe initial film layer is performed at a temperature of about 150° C. toabout 220° C.
 16. The method of claim 12, wherein the PHT comprises: aplurality of trivalent hexahydrotriazine groups having the structure

and a plurality of divalent bridging groups of formula (2):

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ comprises at least 1 carbon and R″comprises at least one carbon, each starred bond of a givenhexahydrotriazine group is covalently linked to a respective one of thedivalent bridging groups, and each starred bond of a given bridginggroup is linked to a respective one of the hexahydrotriazine groups. 17.The method of claim 12, wherein the PHA comprises: a plurality oftrivalent hemiaminal groups having the structure

and a plurality of divalent bridging groups of formula (2):

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ comprises at least 1 carbon and R″comprises at least one carbon, each starred bond of a given hemiaminalgroup is covalently linked to a respective one of the divalent bridginggroups, and each starred bond of a given bridging group is linked to arespective one of the hemiaminal groups.
 18. The method of claim 12,wherein a the first mixture further comprises a diluent monomercomprising one primary aromatic amine group.
 19. The method of claim 18,wherein the diluent monomer is N,N-dimethyl-p-phenylenediamine (DPD),and the PHT comprises a plurality of N,N-dimethyl-p-phenylene groups:

wherein the starred bond is linked to a nitrogen of a hexahydrotriazinegroup.
 20. The method of claim 12, wherein the film layer comprising thePHT has a Young's modulus of about 8 GPa to about 14 GPA.
 21. A filmlayer comprising the polyhexahydrotriazine (PHT) of claim
 1. 22. Adevice comprising the film layer of claim
 21. 23. A device, comprising:a film layer comprising a polyhemiaminal (PHA) disposed on a surface ofa substrate, the PHA comprising a plurality of trivalent hemiaminalgroups having the structure

and a plurality of divalent bridging groups of formula (2):

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ comprises at least 1 carbon and R″comprises at least one carbon, each starred bond of a given hemiaminalgroup is covalently linked to a respective one of the divalent bridginggroups, and each starred bond of a given bridging group is linked to arespective one of the hemiaminal groups.